Here's how to do your own... I didn't discover this, just re-posting for your convenience:

1. Attach the Kindle to a computer using the USB cord.
2. Using Windows Explorer (or whatever you use to see individual files/folders) , you should see a new drive called Kindle under My Computer. Go to that drive. (Or if you have an SD card in your Kindle, you can go to the SD drive.)
3. Create a new folder called “pictures” and a subfolder called “screensavers” under it. Make sure both folder names are all lowercase.
4. Copy your image files into the screensavers folder. Your pictures should be 600×800 pixels and black & white. I’ve only used .jpg files. [[Note: I've used .png files as well]]
5. After copying is complete & it is safe, remove the Kindle from the USB connection.
6. Go to your Home menu and press alt-z. This will create a new book called “screensavers” . Open it. You’ll see each of your pictures. You can advance through them with the prev. page and next page buttons. [[Note: The new book will appear at the END of your book list]]
7. While viewing your pictures, look at the bottom of the Kindle screen. If you see your battery indicator & Menu prompt, press alt-F to go into Full Screen mode.
8. On each photo, press alt-shift-0 (i.e., alt-shift-zero) . After a brief delay, you’ll get a message that your picture has been exported as screensaver. Click Close on the message.
9. Navigate to each of the pictures you want to use and repeat the above step.
10. When you’ve got all your pictures loaded, test them by going into & out of sleep mode. Alt-aA (i.e., alt-font size button) puts it to sleep & wakes it up again. Each time it sleeps, it should use a different image. If you see the same image all the time, relook at Step 3.

If you want to get rid of your custom screen savers, follow these steps:

1. Attach the Kindle to a computer using the USB cord.
2. Using Windows Explorer (or whatever you use to see individual files/folders) , go to your Kindle drive under My Computer.
3. If you see a folder called system, skip this step. If you don’t see the system folder, click on the Tools menu & then Folder Options. Go to the View tab, and look at the Advanced Settings. Under Hidden Files and Folders, select Show Hidden Files and Folders. Click Apply, then OK. You should now see a system folder.
4. Under the system folder, you’ll see a screen_saver folder. Delete all the files that are in that folder.
5. After you disconnect from the USB, you may see your last custom screensaver one more time, but after that it will revert back to the pre-loaded Kindle screensaver images.